Weak Interaction
The weak interaction is one of the four fundamental forces of nature. For neutrinos it is the only one that plays a role — and therefore the reason these particles are so hard to catch.
Four forces, one responsibility
Gravity keeps planets in their orbits. Electromagnetism binds electrons to nuclei. The strong interaction holds atomic nuclei together.
The weak interaction does something else: it transforms particles into one another. It turns a neutron into a proton, a muon into an electron, one quark into another.
It is thus the only force that can change a particle's identity. All the others push, pull or bind — this one transforms.
A neutrino carries neither electric nor colour charge. To electromagnetism and the strong force it is invisible. That leaves only the weak interaction — and gravity, which at such a small mass is of no consequence.
Why the sun burns slowly
The most important service of this force is at the same time the least conspicuous.
The first step of nuclear fusion in the sun consists of two protons merging into a deuterium nucleus. For that, one of the two must convert into a neutron — a process of the weak interaction.
And because this force is so sluggish, that takes time. A single proton in the solar core waits on average billions of years for this step to succeed, although it collides with other protons incessantly.
Precisely this sluggishness makes the sun a power station rather than a bomb. Were the weak interaction stronger, the sun would long since have burnt out — and there would never have been time for life.
Why it is weak
The name misleads. The coupling strength of the weak force is not at all small. What makes it appear weak is the mass of its messenger particles.
Forces are mediated by exchange particles. In electromagnetism that is the massless photon, which reaches arbitrarily far. In the weak interaction they are the W and Z bosons, about eighty times as heavy as a proton.
So heavy an exchange particle can exist only for a minute span of time and accordingly gets hardly anywhere. The range is about 10⁻¹⁸ metres — a thousandth of a proton diameter.
Two particles must therefore come extraordinarily close for anything to happen. That is exactly what makes the cross section so small.
At very high energies this disadvantage loses weight. There the weak force does become comparable in strength to electromagnetism — which is no coincidence but the reason for their unification.
Two kinds of process
Charged currents run via W bosons. In these a particle changes its identity: an electron neutrino becomes an electron, a neutron becomes a proton.
Neutral currents run via the Z boson. Here every particle remains what it is; only energy and momentum are transferred. These processes were discovered in 1973 with the Gargamelle bubble chamber.
The difference is decisive for detection. Because only electron neutrinos react with ordinary matter via charged currents, while neutral currents are open to all kinds, the Sudbury Neutrino Observatory was able to compare the two channels against each other — and so solve the solar neutrino problem.
The peculiarity: left and right
The weak interaction is the only force that distinguishes between left and right. It acts only on left-handed particles and right-handed antiparticles.
Chien-Shiung Wu demonstrated this in 1956. The consequence for the neutrino is its helicity: it is always found left-handed.
This one-sidedness is unexplained to this day. It follows from no deeper principle but is a measured property that has to be built into the theory — one of the few points at which the Standard Model simply describes rather than explains.
Unification
In the 1960s Glashow, Salam and Weinberg showed that the weak interaction and electromagnetism are two sides of the same thing. At very high energies they merge into the electroweak interaction.
That they appear so different to us today is due to the Higgs mechanism: it gives the W and Z bosons their mass and leaves the photon massless. A single force thereby becomes two that could hardly act more differently — one with infinite range, one with almost none.
Related
- Neutrino — the particle that feels only this force
- Cross section — why so little happens
- CEνNS — a neutral-current process put to use
Sources
- S. Weinberg: A Model of Leptons, Physical Review Letters 19, 1264 (1967).
- Particle Data Group: Review of Particle Physics, section Electroweak Model.